This invention relates to the recovery of free metal entrained in the skim or dross which forms on the surface of molten aluminum during melting, holding or treatment.
It is well known that during the processing of molten aluminum, a layer of nonmetallic inpurities (primarily oxides) forms on the surface of the molten aluminum and that much metallic aluminum can be entrained in this layer. Under conventional practice, the skim or dross is removed from the melt surface by mechanical means, such as long hand-held rakes or other implements. When the skim is removed from a furnace, it usually has a free metal content from about 40-85%. The metallic content of the skim depends on many factors, including the alloy composition, if the metal has been degassed and if so, how, and the care exercised by the operator in removing the skim from the melt surface. Wide variations in metal content are common.
If the skim is allowed to stand in a suitable container for a short period of time after it is removed from the melt surface, some molten aluminum will settle to the bottom of the container and can be decanted back into the furnace or poured into a mold. However, much metallic aluminum remains intimately mixed within the skim in the form of small globules or particles which will not readily separate from the mass of skim.
Due to the high skim temperature when the skim is removed from the molten aluminum surface, the molten aluminum in the skim mass frequently begins to rapidly oxidize (commonly termed thermiting), consuming a considerable amount of the free metal in the dross. A frequently used rule of thumb is that 1% of the molten aluminum in hot skim is oxidized for every minute the skim is exposed to the atmosphere. If allowed to continue, essentially all of the molten aluminum in the skim would be consumed.
Several methods have been used in the past to reclaim the free aluminum entrained in the skim. One method has been to rapidly cool the skim (to avoid oxidation of the free metal), grind the skim in a ball mill (or other suitable device) and then separate the ground material into a coarse (metallic) fraction and a fine (nonmetallic) fraction. This method is simple and inexpensive, but the overall metal recovery is usually very poor. A second method has been to mix the skim into a molten salt bath which frequently contained fluorides. The molten salt causes the coalescence of the molten aluminum globules in the skim and the separation of the molten metal from the oxide portion of the skim. Although a very high metal recovery is obtained, the salts used in this process are expensive and present environmental pollution laws make the disposal of the spent salts difficult. A third method involved mixing or agitating the hot skim in some manner, such as with a mixing blade or paddle, to mechanically separate the molten aluminum from the hot skim. However, the mixing or beating action of the blade was not very efficient in causing the coalescence of the molten aluminum globules and, moreover, it entrained much oxide into the molten aluminum which did coalesce. Generally, temperature control in such devices was difficult. Moreover, installations having such mechanical devices have been usually limited to treating small quantities of skim and are of little value in handling the volume of skim generated at large aluminum plants.
A modification of the above third method is described in U.S. Pat. No. 2,481,591 (Hellman et al) wherein the hot skim is tumbled in a rotating furnace to cause a partial oxidation of the molten metal therein. It is alleged by the patentees that the extremely high temperature (1550.degree.-2300.degree. F. generated by the oxidizing free metal in the skim caused a breakdown between the oxide and the free metal and resulted in a good separation between the metal and the oxide during the tumbling of the skim in the rotary furnace. Although good metal recoveries are alleged, it has been found that much metal is consumed to generate the extremely high temperatures required by this process and, further, the process was fraught with operational problems. For example, at skim temperatures above 1500.degree. F. the oxidation of molten aluminum is very difficult to control due to the rapid and highly exothermic nature of the oxidation reaction. Such uncontrolled oxidation generates temperatures in excess of 3000.degree. F. which can result in the early destruction of the refractory lining and even the destruction of the furnace itself. Additionally, the high temperatures cause an oxide build-up on the furnace lining which interferes with the tumbling of the skim and the discharge of molten aluminum and spent skim from the furnace. The build-up apparently results from the sintering of oxides to the refractory lining of the furnace. Excessively high temperatures are also characterized by the generation of copious amounts of dust which considerably increase the capital costs for dust collection equipment. Another problem often encountered with this high temperature process is the formation of large sintered masses of spent skim in the rotary furnace which are difficult to discharge because of their size and, when removed from the furnace, are difficult to handle and dispose of.
To minimize some of the problems attendant with the process described by Hellman et al in U.S. Pat. No. 2,481,591, it has been suggested by Stroup et al in U.S. Pat. No. 2,754,199 to maintain an AlCl.sub.3 atmosphere over the tumbling mass of skim to prevent excessive skim temperatues. However, the addition of chlorine (or other halide) to the rotary furnace is not desirable due to the expense and trouble of treating an effluent containing a chloride (or other halide) gas or mist. The AlCl.sub.3 formed by the chlorine quickly reacts with any moisture present in the atmosphere to form HCl and a submicroscopic particulate Al.sub.2 O.sub.3 which are very difficult to capture by conventional means. Use of a sealed furnace would avoid many of the problems of processing the skim in a halide containing gas but a sealed rotary furnace for large industrial applications would be impractical in most instances.
It is against this background that the present invention was developed.